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Curious about the I.R.E. Standard Dummy Antenna
Pete, I forgot to mention the reason I believe the IRE dummy antenna
tries to look like a 400-ohm resistive load at shortwave instead of a certain length of wire. You can see why he http://users.tns.net/~bb/antlen.gif This is an old RCA chart showing the behavior of an end-fed wire for lengths to 125 feet. It indicates the many resonances and antiresonances a short wire can exhibit. Since the antenna impedance and the resulting performance vary so markedly with wire length, and because it would be unrealistic to expect all listeners to use one particular length, I think the dummy antenna is just intended to roughly match the input impedance of a typical radio. I'm not sure about consumer radios, but old communications receivers all seem to have a specified input impedance of either 300 or 400 ohms. On the broadcast band, any wire shorter than about 150 feet will look capacitive. It won't exhibit the resonances you see in the chart at shortwave. So even if the wire is shorter than the 100-foot length the dummy antenna seems to model and exhibits a higher capacitive reactance, it won't affect the radio's RF tracking that much. On the broadcast band, most radios seem to use rather loose antenna coupling to minimize mistracking. This allows them to accomodate antennas of various length. There is an interesting discussion about antenna coupling strategies in the Radiotron Designer's Handbook. Brian |
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